1. Field of the Invention
This invention relates to data acquisition systems and more particularly to data acquisition systems in which individual signals are selected from a multi-line input and displayed on a cathode ray tube such as is done in continuous tone frequency modulated sonar systems.
2. Discussion of Related Art
In a conventional continuous tone frequency modulated (CTFM) sonar, a transmitter having a transducer ensonifies the water about the transducer, and echoes from objects in the beam of the transducer are converted to frequencies which are proportional to the range of the objects from the transducer. The recovered information is applied to a bank of filters, each tuned to pass a narrow band of frequencies, ranging from the lowest to the highest frequencies of interest. For example, forty filters, each having a 50 hz bandwidth and ranging from 500 hertz to 2500 hertz may be used. The outputs of the filters are rectified and applied to a data acquisition circuit which comprises a multiplexer. The multiplexer is stepped in sequence by a fixed clock frequency system so that the output of each filter channel is transferred in turn to the output of the multiplexer for eventual display or further processing. An equal amount of time is allowed for the multiplexer to dwell on each channel regardless of the amount of useful signal information contained in the channel. Such a system is shown and described in U.S. Pat. No. 3,939,463 to Kelly et al., the disclosure of which is incorporated herein by reference. This system will be described in further detail hereinafter with reference to FIG. 2 of the attached drawings.
A difficulty with the Kelly et al. system is that during any particular scan, only a limited number of the channels presented to the data acquisition system contain useful information. Because all the channels are accessed for an equal amount of time, those channels containing only noise are displayed on the screen with equal priority to those channels containing useful information. Consequently, the display of an object sensed by the sonar system may appear to be very weak or flicker, thus making the display very difficult to read. Consequently, a need has developed for increasing the clarity of the display in such a sonar system.
Other systems which use data acquisition circuits are also known. For example, U.S. Pat. No. 4,118,701 to Brey et al. shows a radar system wherein there is transmitted a signal of constant amplitude which linearly varies with frequency at a selected rate for some selected range of detection. In the detection process, the received signal is processed and used to modulate a frequency responsive light deflector through which is passed a narrow beam of light. A plurality of photosensitive elements are positioned to incrementally pick up the beam over a calibrated deflection range. The output of each photosensitive element is fed through a threshold detector which provides an output when there appears a signal greater than uncorrelated noise.
U.S. Pat. No. 3,789,350 to Rolle shows an ultrasonic imaging system having a plurality of channels. Each channel produces a signal, which signals are individually sampled by multiplexer gates in accordance with an order of succession determined by a multiplexer logic. The peak of each signal during each pulse period is detected and held so that it may be displayed during the next succeeding pulse period or a length of time greater than that for which it could be displayed during the pulse period in which it is generated.
U.S. Pat. No. 3,701,089 to Cowan discloses a display for multi-channel sonar systems in which outputs from the sonar receiver are converted to video signals which are coupled directly to the grid or cathode of a cathode ray tube in order to improve the display associated with low frequency signals.
U.S. Pat. No. 4,259,734 to Hormel discloses a water depth measuring system in which pulse signals are transmitted and received. The signals may be of any one of four different frequencies. In the receiver circuit, AND gates are enabled in order to be responsive only to echoes expected in a given time window.